The present application relates to a secondary battery that includes a spirally wound electrode body in which a positive electrode and a negative electrode are laminated and spirally wound via a separator, and a battery pack, an electronic apparatus, an electric tool, an electric vehicle, and a power storage system that use such a secondary battery.
In recent years, electronic apparatuses typified by mobile terminal apparatuses and the like have been popularized, and there is demand for such apparatuses to be further miniaturized, lightened, and to have a longer service life. Accordingly, the development of a battery, particularly a secondary battery that is small, light, and able to obtain a high energy density as a power source has been pursued. Further, with secondary batteries, application for uses in large sizes such as for automobiles is also being considered, and wide applications for other uses is also being considered.
As a secondary battery, the use of a variety of elements as the carrier (material that reciprocates between the positive electrode and the negative electrode during charging and recharging) is being researched. Among such secondary batteries, there are great expectations on a secondary battery that uses lithium as the carrier, specifically, a lithium ion secondary battery that uses the absorption and the discharge of the lithium as a charging and discharging reaction. The reason is that it is possible to obtain a higher energy density than a lead battery or a nickel cadmium battery.
A secondary battery includes a spirally wound electrode body that is the so-called cell element, and the spirally wound electrode body is composed by a positive electrode and a negative electrode that are laminated via a separator and spirally wound. The positive electrode includes a positive electrode active material layer that is formed on a positive electrode collector, and the negative electrode includes a negative electrode active material layer that is formed on a negative electrode collector.
Incidentally, recently, accompanying the rapid increase in performance and functionality of electronic apparatuses and the like, there is a strong demand to further improve the performance of secondary batteries, particularly to increase capacity. Accompanying such a demand, in order to increase the charging and discharging capacity of a positive electrode and a negative electrode, if a high capacity material is used as the negative electrode active material that is included in the negative electrode active material layer, there is accordingly cause to increase the thickness of the positive electrode active material layer. Further, in order to increase the volume occupied by the positive electrode and the negative electrode in the secondary battery, if the thickness of the positive electrode collector and the separator are reduced, the thicknesses of the positive electrode active material layer and the negative electrode active material layer are accordingly increased.
However, if the thickness of the positive electrode active material layer is increased, the flexibility of the positive electrode active material layer thereof decreases. Therefore, if the positive electrode is spirally wound around a coil core rod along with the negative electrode and the like when creating the spirally wound electrode body, the positive electrode in the vicinity of the center where the radius of curvature is small becomes prone to fracturing. Such a tendency becomes particularly striking the smaller the outer diameter of the coil core rod is made in order to increase the volume occupied by the positive electrode and the negative electrode within the secondary battery.
Therefore, in order to suppress fracturing of the positive electrode when coiling, a variety of countermeasures and related techniques have been proposed. Specifically, when creating a flat coil group composed of substantially straight portions and curved portions by coiling a positive electrode plate or the like, causing a solvent to contact portions that correspond to the curved portions of the positive electrode plate and lowering the positive electrode plate density of the curved portions to be lower than the positive electrode plate density of the substantially straight portions has been proposed (for example, refer to Japanese Unexamined Patent Application Publication No. 2005-310617). Causing the application thickness of an electrode compound that is applied to the inside to be thinner than the application thickness of the electrode compound that is applied to the outside when coiling a sheet-shaped electrode is proposed (for example, refer to Japanese Unexamined Patent Application Publication No. 08-130035). Including a predetermined amount of a copolymer of vinylidene fluoride, tetrafluoroethylene, and hexafluoropropylene as a binder on the positive electrode active material layer has been proposed (for example, refer to Japanese Unexamined Patent Application Publication No. 2006-059771). Out of the binder layers (inner circumference layer and outer circumference layer) that are provided on both sides of the collector, causing the thickness of the outer circumference layer to be greater than the thickness of the inner circumference layer and causing the active material amount of the outer circumference side to be greater than the active material amount of the inner circumference layer has been proposed (for example, refer to Japanese Patent No. 3131976). In a positive electrode collector after recharging one or more times, causing the coefficient of extension until fracturing in the coiling direction to be equal to or greater than 3% has been proposed (for example, refer to Japanese Unexamined Patent Application Publication No. 2006-134762).
Further, out of an outer surface positive electrode active material layer and an inner surface positive electrode active material layer that are provided on both sides of the positive electrode collector, causing the thickness of the inner surface positive electrode active material layer to be less than the thickness of the outer surface electrode active material layer and providing an outer surface active material region in which only the outer surface active material layer is provided on a position that overlaps a lead on the coil center side of the positive electrode has been proposed (for example, refer to Japanese Unexamined Patent Application Publication No. 2008-004531). Out of an inner side positive electrode active material layer and an outer side positive electrode active material layer that are provided on the positive electrode body, causing the center angle of an end portion on a coil center side of the outer side positive electrode active material layer and an end portion on a coil center side of the inner side positive electrode active material layer to the coil center to be equal to or greater than 72° and providing a positive electrode lead so as to avoid a region in which the center angle from the end portion of the coil center side of the inner side positive electrode active material layer to be within 30° in the coil direction and within 30° in the opposite direction to the coil direction has been proposed (for example, refer to Japanese Unexamined Patent Application Publication No. 2006-134763). Providing an outer circumference side opposing region in which the negative electrode active material layer and the positive electrode active material layer are opposing in only the outer circumference surface side to be within a range of equal to or more than 2 revolutions and equal to or less than 3.25 revolutions on the coil center side of the negative electrode has been proposed (for example, refer to Japanese Unexamined Patent Application Publication No. 2006-024464).